I'm wondering because I've seen data which shows that Black Americans have over 25% of their White DNA from a White Female Ancestor, which surprised me as I had previously believed that only White Men at the time engaged in sexual abuse against Black Women.
In this case, what was the history of White Women sexually exploiting Black Men during this era?
I'll let someone more knowledgeable about slavery than I am answer your question, but I'd like to offer a cautionary note about taking these percentages too seriously.
The fact that African Americans have 25% of their European ancestry from European female ancestors does not mean that these percentages reflect the frequency of African-European pairings by sex. The genetics of modern people is strongly influenced by successive generations of survivor bias. In other words, not everyone lives long enough to reproduce, not everyone has the same number of children, not all children have an equal chance to survive to adulthood and have children of their own. Any biases in these numbers get amplified over time.
Here's the actual data for African American ancestry from the paper:
| Ancestor | Ancestry in Current Population |
|---|---|
| African Male | 31.0% |
| African Female | 42.2% |
| European Male | 18.8% |
| European Female | 5.2% |
| Native American Male | 0.2% |
| Native American Female | 0.6% |
| Unclassified | 2.0% |
| Total | 100% |
While it's true that European female ancestry is quite high (5.2%, or a bit more than a quarter of total European ancestry), notice that African female ancestry is also higher than African male ancestry (42.2% versus 31%). This shows that either African women reproduced more than African men, or that more of their descendants survived to the present day, or both.
In fact, there may be reasons for both. Humans all over the world have more ancestry from female ancestors than from male, or to be more specific, we have DNA from a comparatively larger number of women who lived in the past than we have from men. This is because significantly more women than men managed to reproduce during their lifetimes. To put it in other words, the number of children per woman is more evenly distributed, but a relatively fewer number of men fathered a lot of children while other men had none. This can have many causes; polygyny is often cited as one historically. In the context of slavery, it may be that many African men were sent to work in the fields, which is harder work and less safe than being a house-slave, so perhaps fewer of them got the chance to reproduce.
Then there is the difference in survival rates of the children. If the children of some pairings have a better chance to survive than others, over time this will amplify their lineage quite dramatically. If you make the assumption that European mothers had a higher social status than African mothers in a racist society, then it's not surprising that more of their children survived to adulthood than the children of African mothers. Attitudes towards children may also play a role. For example, if the European father sees the pairing as just casual sex with an African slave, he may not feel much responsibility for the welfare of the child.
In short, while clearly European women had children with African men, how common this was is probably not accurately represented by the numbers you cite. It's quite easy for secondary effects to amplify a lineage or to bring an end to it, so when you look at data several generations later, you see a complicated picture that has several contributory factors.
More can always be said, but this older answer might be of interest for you.